Catalytic coating and method of manufacturing thereof

ABSTRACT

The invention relates to a catalytic coating of surfaces of valve metals, for example titanium, suitable for operation in highly aggressive electrolytic environments such as electrolysis cells of hydrochloric acid. The coating may be used as catalytic activation of electrodes, for example for anodic evolution of chlorine, or for protection from crevice corrosion of flanges and other elements of electrolysers subject to stagnation of liquid.

FIELD OF THE INVENTION

The invention relates to a catalytic coating of valve metal articles suitable for use in highly aggressive electrolytic environments, for example in hydrochloric acid electrolysis cells.

BACKGROUND OF THE INVENTION

Hydrochloric acid electrolysis is an electrochemical process gaining increasing interest at present, being hydrochloric acid the typical by-product of all major industrial processes making use of chlorine: the increase in the production capacity of plants of new conception entails the formation of significant amounts of acid, whose placement on the market presents significant difficulties. The electrolysis of the acid, typically carried out in two-compartment electrolytic cells separated by an ion-exchange membrane, leads to the formation of chlorine at the anode compartment, which can be recycled upstream resulting in a substantially closed cycle of negligible environmental impact. The construction materials of the anodic compartment must be capable of withstanding an aggressive environment combining acidity, humid chlorine and anodic polarisation while retaining a suitable electrical conductivity; for such purpose, valve metals such as titanium, niobium and zirconium are preferably employed, optionally alloyed titanium being the most common example for reasons of cost and ease of machining. Titanium alloys containing nickel, chromium and small amounts of noble metals such as ruthenium and palladium, like the AKOT® alloy commercialised by Kobe Steel, are for instance of widespread use. The anodes whereon the anodic evolution of chlorine is carried out consist for example of a valve metal article such as a titanium alloy substrate coated with a suitable catalyst, typically consisting of a mixture of oxides of titanium and ruthenium, capable of lowering the overvoltage of the anodic discharge of chlorine. The same type of coating is also used to protect from corrosion some components of the anodic compartment not directly involved in the evolution of chlorine, with particular reference to interstitial areas subject to electrolyte stagnation. The lack of a sufficient electrolyte renewal may in fact lead to a local discontinuity of the passivation layer directed at protecting the valve metal, triggering corrosion phenomena, which are the more dangerous the more they are localised in small areas. An example of areas subject to delimiting interstices is given by the peripheral flanges of both the anodic and the cathodic compartment, whereupon sealing gaskets are typically assembled. In the most favourable cases experienced in the industrial practice, titanium alloys coated with catalytic formulations based on oxides of ruthenium and titanium may ensure a continuous operation in a hydrochloric acid electrolysis plant in the range of 24 to 48 months, before corrosion problems leading to deactivation of the anode structure and/or leakage of cell elements in the flange area take place. For the sake of improving the competitiveness of the industrial hydrochloric acid electrolysis process it is necessary to further increase the useful lifetime of these components.

SUMMARY OF THE INVENTION

Various aspects of the present invention are set out in the accompanying claims.

Under one aspect, the invention relates to a coating of valve metal surfaces including a titanium-free catalytic layer and consisting of the mixture of two phases, namely an amorphous phase of Ta₂O₅ in admixture with a tetragonal ditetragonal dipyramidal crystalline phase containing RuO₂, optionally in solid solution with SnO₂ The inventors have in fact observed that titanium-free coatings are more resistant to chloride attack in acidic solution, presumably because titanium oxides—whose function in a combination with ruthenium dioxide is to act as film-forming component—are present as a mixture of crystalline phases including an anatase TiO₂ phase, substantially weaker than the others. The inventors have also observed that mixtures of oxides of tantalum and ruthenium in an amorphous phase do not contribute to solving the problem in a decisive manner, even if completely free from titanium. When, however, the coating is formed from a mixture of RuO₂ in the typical crystalline form similar to rutile (i.e. tetragonal ditetragonal dipyramidal) and Ta₂O₅ in a basically amorphous phase, the stability of the coating to acid attack is greatly increased. As a further advantage, the overvoltage of the coating towards anodic chlorine evolution is surprisingly reduced. In one embodiment, the weight ratio between the amorphous phase of Ta₂O₅ and the crystalline phase is between 0.25 and 4, which defines the best range of functioning of the invention. In one embodiment, the RuO₂ component in the tetragonal ditetragonal dipyramidal crystalline phase is partially replaced by SnO₂ (cassiterite). The two dioxides of tin and of ruthenium, whose tetragonal ditetragonal dipyramidal crystalline form turns out to be the most stable, are capable of forming solid solutions in any weight ratio; in one embodiment, the Ru to Sn weight ratio in the tetragonal ditetragonal dipyramidal crystalline phase of the coating ranges between 0.5 and 2, which gives the best results in terms of protection of the substrate as well as of catalytic activity of the coating. In one embodiment, the coating comprises two distinct catalytic layers, one as hereinbefore described in direct contact with the valve metal substrate coupled to an outermost one overlaid thereto with a higher content of ruthenium oxide. This can have the advantage of enhancing on one hand the protective function at the substrate surface and on the other hand the catalytic and conductive properties of the outermost layer, as required for example in the case wherein the coating is used for the catalytic activation of an anodic structure whose outer surface is in direct contact with the electrolyte. In one embodiment, the inner catalytic layer has a weight ratio of amorphous Ta₂O₅ phase to RuO₂-containing crystalline phase (optionally including SnO₂) ranging between 0.25 and 2.5 and the outer catalytic layer consists of an amorphous phase of Ta₂O₅ mixed with a tetragonal ditetragonal dipyramidal crystalline phase of RuO₂ with a Ru to Ta weight ratio between 3 and 5. In one embodiment, between the coating as hereinbefore described—in one or two coats—and the substrate there is interposed a further protective pre-layer consisting of a mixture of oxides of titanium and tantalum. This can have the advantage of improving the anchoring of the catalytic layer to the substrate, at the expense of a resistive penalty deriving from the modest electrical conductivity of mixtures of titanium and tantalum oxides. The magnitude of such resistive penalty can be however very limited, provided the pre-layer has a suitably limited thickness. A total loading of titanium and tantalum oxides of 0.6 to 4 g/m² is a suitable value for a pre-layer to be combined with a catalytic layer containing 20 g/m² of total oxides.

In another aspect, the invention relates to a method for the manufacturing of a coating as hereinbefore described comprising the optional application of a solution of titanium and tantalum compounds, for example TiOCl₂, TiCl₃ and TaCl₅, to a valve metal substrate in one or more coats, with subsequent thermal decomposition after each coat; the application of a solution of compounds of tantalum, ruthenium and optionally tin in one or more coats, with subsequent thermal decomposition after each coat, until obtaining a first catalytic layer; the optional application of a solution of compounds of tantalum and ruthenium upon the first catalytic layer with subsequent thermal decomposition after each coat, until obtaining a second catalytic layer. In one embodiment, the compounds of ruthenium and tin applied in view of the subsequent thermal decomposition are hydroxyacetochloride complexes; this can have the advantage of obtaining more regular and compact layers, having a more homogeneous composition, compared to hydrochloric or other precursors. The thermal decomposition step after each coat can be effected between 350 and 600° C., depending on the selected precursor compounds. In the case of decomposition of mixtures of precursors consisting of tantalum chloride and hydroxyacetochloride complexes of ruthenium and optionally of tin, thermal decomposition may for example be carried out between 450 and 550° C.

The following examples are included to demonstrate particular embodiments of the invention, whose practicability has been largely verified in the claimed range of values. It should be appreciated by those of skill in the art that the compositions and techniques disclosed in the examples which follow represent compositions and techniques discovered by the inventors to function well in the practice of the invention; however, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the scope of the invention.

EXAMPLE 1

A 1 mm thick AKOT® titanium alloy mesh was degreased with acetone in a ultrasonic bath and etched in 20% HCl at boiling temperature for 15 minutes. The mesh was cut into a plurality of pieces of 10 cm×10 cm size for the subsequent preparation of electrode samples.

A solution of precursors for the preparation of the protective pre-layer was obtained by mixing 150 g/l of TiOCl₂ and 50 g/l of TaCl₅ in 10% wt. hydrochloric acid.

A first series of catalytic solutions was obtained by mixing 20% by weight RuCl₃ and 50 g/l TaCl₅ in 10% wt. hydrochloric acid according to various proportions.

Solutions of hydroxyacetochloride complexes of Ru (0.9 M) and Sn (1.65 M) were obtained by dissolving the corresponding chlorides in 10% vol. aqueous acetic acid, evaporating the solvent, taking up with 10% aqueous acetic acid with subsequent evaporation of the solvent for two more times, finally dissolving the product again in 10% aqueous acetic acid to obtain the specified concentration.

A second series of catalytic solutions was obtained by mixing the hydroxyacetochloride complexes of Ru and Sn according to various proportions.

Electrode samples were obtained at different formulations with the following procedure:

-   -   a protective pre-layer was applied to the samples cut out of the         titanium mesh by brushing the solution containing TiOCl₂ and         TaCl₅ precursors in two coats, with subsequent drying at 50° C.         for 5 minutes and thermal decomposition treatment at 515° C. for         5 minutes after each coat, until obtaining a deposit of oxides         of tantalum and titanium with a loading of about 1 g/m²;     -   catalytic layers of various formulations were applied upon the         protective pre-layer of the above samples by alternatively         applying catalytic solutions of the first or of the second         series. The catalytic solutions of the first series were applied         by brushing in 8-10 coats and subjected to subsequent drying at         50° C. for 10 minutes and thermal decomposition treatment at         500° C. for 5 minutes after each coat, until obtaining a deposit         of oxides of tantalum and ruthenium with a total ruthenium         loading of about 20 g/m². At the end of the thermal         decomposition process, the electrodes were subjected to a         subsequent thermal cycle of 2 hours at 500° C., until obtaining         a crystalline tetragonal ditetragonal dipyramidal ruthenium         dioxide phase mixed with the amorphous tantalum oxide phase, as         verified by means of a subsequent XRD investigation. Some         samples of electrodes thus obtained are indicated in Table 1 as         RuTa type. The catalytic solutions of the second series have         been applied by brushing in 8-10 coats and subjected to         subsequent drying at 60° C. for 10 minutes and thermal         decomposition treatment at 500° C. for 5 minutes after each         coat, until obtaining a deposit of oxides of tantalum, tin and         ruthenium with a total ruthenium loading of about 20 g/m². Also         in this case, at the end of the thermal decomposition process,         the electrodes were subjected to a subsequent thermal cycle of 2         hours at 500° C., until obtaining a solid solution of ruthenium         dioxide and tin dioxide in a crystalline tetragonal ditetragonal         dipyramidal phase mixed with the amorphous phase of tantalum         oxide, as verified by a subsequent XRD investigation. Some         samples of electrodes thus obtained are indicated in Table 1 as         RuTaSn type;     -   other electrode samples provided with a catalytic coating         consisting of two layers were obtained by alternatively applying         catalytic solutions of the first or of the second series. The         catalytic solutions of the first series were applied by brushing         in 6-7 coats and subjected to subsequent drying at 50° C. for 5         minutes and thermal decomposition treatment at 500° C. for 5         minutes after each coat, until obtaining a first deposit of         oxides of ruthenium and tantalum; a subsequent solution of the         first type with a Ru to Ta weight ratio equal to 4 was         subsequently applied by brushing in 2 coats and subjected to the         same drying and thermal decomposition cycle after each coat,         until obtaining a total ruthenium loading of approximately 20         g/m². At the end of the thermal decomposition process, the         electrodes were subjected to a subsequent thermal cycle of 2         hours at 500° C., until obtaining a crystalline tetragonal         ditetragonal dipyramidal phase of ruthenium dioxide mixed with         the amorphous phase of tantalum oxide, as verified by a         subsequent XRD investigation. Some samples of electrodes thus         obtained are indicated in Table 1 as RuTa_TOP type. The         catalytic solutions of the second series were applied by         brushing in 6-7 coats and subjected to subsequent drying at         60° C. for 5 minutes and thermal decomposition treatment at         500° C. for 10 minutes after each coat, until obtaining a         deposit of oxides of tantalum, tin and ruthenium; a deposit of         oxides of ruthenium and tantalum, obtained upon brushing in 2         coats of a solution of the first type with a Ru to Ta weight         ratio equal to 4, subjected to drying at 50° C. for 5 minutes         and thermal decomposition at 500° C. for 10 minutes after each         coat, was overlaid thereto, until obtaining a catalytic coating         in two layers with a total ruthenium loading of about 20 g/m².         At the end of the thermal decomposition process, the electrodes         were subjected to a subsequent thermal cycle of 2 hours at 500°         C., until obtaining a solid solution of ruthenium dioxide and         tin dioxide in a tetragonal ditetragonal dipyramidal crystalline         phase mixed with the amorphous phase of tantalum oxide in the         inner layer and of a tetragonal ditetragonal dipyramidal         ruthenium dioxide crystal phase mixed with the amorphous phase         of tantalum oxide in the outer layer, as verified by a         subsequent investigation by XRD. Some samples of electrodes thus         obtained are indicated in Table 1 as RuTaSn_TOP type.

TABLE 1 Weight composition Weight composition # Sample Type 1st layer 2nd layer 1 RuTa Ru Ta 25 75 — 2 RuTa Ru Ta 60 40 — 3 RuTa Ru Ta 80 20 — 4 RuTaSn Ru Ta 27 41 Sn 32 — 5 RuTaSn Ru Ta 20 70 Sn 10 — 6 RuTaSn Ru Ta 60 20 Sn 20 — 7 RuTa_TOP Ru Ta 40 60 Ru Ta 80 20 8 RuTa_TOP Ru Ta 70 30 Ru Ta 80 20 9 RuTaSn_TOP Ru Ta 20 70 Sn 10 Ru Ta 80 20 10 RuTaSn_TOP Ru Ta 60 20 Sn 20 Ru Ta 80 20

COUNTEREXAMPLE 1

A 1 mm thick AKOT® titanium alloy mesh was degreased with acetone in a ultrasonic bath and etched in 20% HCl at boiling temperature for 15 minutes. The mesh was cut into a plurality of pieces of 10 cm×10 cm size for the subsequent preparation of electrode samples.

A solution of precursors for the preparation of the protective pre-layer was obtained by mixing 150 g/l of TiOCl₂ and 50 g/l of TaCl₅ in 10% hydrochloric acid. A series of catalytic solutions was obtained by mixing 20% by weight RuCl₃ and 150 g/l TiOCl₂ in 10% hydrochloric acid according to various proportions.

-   -   a protective pre-layer was applied to the samples cut out of the         titanium mesh as in the case of Example 1     -   catalytic layers of various formulations were applied on the         protective pre-layer of the above samples by brushing the above         catalytic solutions in 8-10 coats and subjected to subsequent         drying at 50° C. for 5 minutes and thermal decomposition         treatment at 500° C. for 5 minutes after each coat, until         obtaining a deposit of oxides of ruthenium and titanium with a         total ruthenium loading of about 20 g/m². At the end of the         thermal decomposition process, the electrodes were subjected to         a subsequent thermal cycle of 2 hours at 500° C. Some samples of         electrodes thus obtained are indicated in Table 2 as RuTi type.

TABLE 2 # Sample Type Weight composition C1 RuTi Ru 25 Ti 75 C2 RuTi Ru 60 Ti 40 C3 RuTi Ru 80 Ti 20

EXAMPLE 2

The electrode samples shown in the table were subjected to a test of standard potential under anodic evolution of chlorine at the current density of 3 kA/m², in 15% wt. HCl at a temperature of 60° C. The potential data obtained are reported in Table 3 (SEP). The table shows also the related data of an accelerated lifetime test, expressed in terms of hours of operation before deactivation under anodic evolution of chlorine at the current density of 6 kA/m², in 20% wt. HCl at a temperature of 60° C., using a zirconium cathode as counterelectrode. The deactivation of the electrode is defined by a 1 V increase in the cell with respect to the initial value.

TABLE 3 # Sample SEP (V) Lifetime (hours) 1 1.175 4020 2 1.168 4880 3 1.160 4700 4 1.170 5770 5 1.173 5060 6 1.166 5980 7 1.161 4550 8 1.154 4520 9 1.161 5690 10  1.158 5540 C1 1.190 3660 C2 1.183 3900 C3 1.183 3280

EXAMPLE 3

Duplicates of electrode samples 2, 6 and C2 were subjected to a corrosion test which simulates the crevice corrosion conditions that can occur on the flanges of electrolysers for the production of chlorine or other occluded zones. A first series of samples was immersed in a known volume of 20% wt. HCl at 45° C. under nitrogen stream, to simulate electrolyte stagnation conditions; a second (control) series was immersed in the same volume of 20% wt. HCl at 40° C. under a stream of oxygen, in order to maintain passivation. In both cases, the concentration of chromium and nickel released from the substrate in the course of 24 hours was detected: for samples 2 and 6, the concentration of both metals in the volume of HCl was less than 2 mg/l, while sample C2 showed concentrations slightly higher than 2 mg/l of Cr and 4 mg/l of Ni under a stream of oxygen, which increased significantly under a stream of nitrogen (up to 6.5 mg/l for nickel).

The test was repeated with another set of samples, confirming a substantial increase in the corrosion resistance for the formulations of the invention.

The foregoing description shall not be intended as limiting the invention, which may be used according to different embodiments without departing from the scopes thereof, and whose extent is solely defined by the appended claims.

Throughout the description and claims of the present application, the term “comprise” and variations thereof such as “comprising” and “comprises” are not intended to exclude the presence of other elements, components or additional process steps. The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application. 

1. A coated valve metal substrate having a coating comprising a titanium-free first catalytic layer consisting of an amorphous phase of Ta₂O₅ in admixture with a tetragonal ditetragonal dipyramidal crystalline phase consisting of either RuO₂ or a solid solution of RuO₂ and SnO₂, said first catalytic layer obtained from a solution of compounds of tantalum, ruthenium and optionally tin in one or more coats, with subsequent thermal decomposition after each coat.
 2. The coating according to claim 1 wherein the weight ratio of said amorphous phase to said crystalline phase ranges from 0.25 to
 4. 3. The coating according to claim 1 wherein the Ru to Sn weight ratio in said crystalline phase ranges from 0.5 to
 2. 4. The coating according to claim 1 comprising a second catalytic layer applied externally to said first catalytic layer, wherein said first catalytic layer has a weight ratio of said amorphous phase to said crystalline phase ranging from 0.25 to 2.5 and said second catalytic layer consists of an amorphous phase of Ta₂O₅ mixed with a tetragonal ditetragonal dipyramidal crystalline phase of RuO₂ with a Ru to Ta weight ratio ranging from 3 to
 5. 5. The coating according to claim 1 comprising a protective pre-layer consisting of a mixture of oxides of titanium and tantalum interposed between the valve metal surface and said first catalytic layer.
 6. The coating according to claim 1 applied to a substrate of titanium or titanium alloy.
 7. The coating according to claim 6 wherein said substrate is an anode substrate or a flange of a chlorine-producing electrolyser.
 8. The coating according to claim 7 wherein said electrolyser is a hydrochloric acid electrolyser.
 9. A method for manufacturing a coating according to claim 1 comprising the following simultaneous or sequential steps: optionally applying a solution of compounds of titanium and tantalum to a valve metal substrate in one or more coats, with subsequent thermal decomposition after each coat; applying of a solution of compounds of tantalum, ruthenium and optionally tin in one or more coats, with subsequent thermal decomposition after each coat, until obtaining a first catalytic layer; and optionally applying of a solution of compounds of tantalum and ruthenium to said first catalytic layer in one or more coats, with subsequent thermal decomposition after each coat, until obtaining of a second catalytic layer.
 10. The method according to claim 9, wherein said compounds of ruthenium and tin are hydroxyacetochloride complexes. 